Research continues to emerge to support the potential benefits of fasting for people with certain (though not all) types of cancer. Fasting is being studied as a way to reduce the risk of metabolic syndrome and diabetes. It is being studied for autoimmune and cardiovascular diseases too. Health experts who study the effects of fasting believe it may be able to:

dampen inflammation in the body

improve immune function overall

decrease autoimmunity, a set of immune responses by the body against it’s own healthy tissues and cells

decrease risk of neurodegenerative diseases, which can lead to cognitive decline with age, such as what happens in Alzheimer’s disease

Intermittent Fasting

I referred to fasting above as “intermittent fasting.” There are several different types of intermittent fasting, which are well defined by an excellent article on fasting in The Scientist. Here is a summary of their definitions, with some explanatory additions from me:

Alternate-day fasting: Subjects eat a standard healthy diet every other day. For humans, non-eating days typically consist of one small meal of around 500 calories, amounting to a dietary energy reduction of approximately 65 percent to 80 percent of usual intake. This means “non-eating days” aren’t 100% calorie free, though calories are severely restricted on “fasting” days.

Periodic fasting: This fast is undertaken anywhere from once a month to once a year. For a period of at least five days, food is avoided completely or subjects eat a modified “fasting-mimicking diet” (FMD), which steps down energy intake over the fasting period, and is low in carbohydrates, proteins, and calories. The FMD approach appears to provide much of the same benefit of full fasting (no calories from any source), with one advantage: Being able to have some calories each day (mostly from fat) can be easier for people to follow than a “water-only” fast.

Time-restricted feeding: Calories are not restricted, and dietary composition is not altered. But eating is confined to a window of 8, 10, or 12 hours per day. For example, you’re confining your eating to 8 hours out of a 24 hour day, and the last time you ate was dinner at 8pm the evening prior, your first meal of the day would be noon.

5:2 diet: A person eats five days of the week and abstains from eating, or eats just a very small amount the other two days. For example eating on Monday, Wednesday, Friday, Saturday, and Sunday and fasting on Tuesday and Thursday would be a 5:2 diet. On fasting days, a person can eat one small, 500-calorie meal, cutting dietary energy by about 65 percent to 80 percent on the fast days. As with periodic fasting and alternate day fasting, the fasting days can be a complete fast from all calories, or may follow a very low calorie FMD – Fasting Mimicking Diet.

How is Fasting Related to Refeeding Syndrome?

I never gave much thought about how best to break a fast until I stumbled onto Nat Eliason’s blog on his experience with a five day fast. I often search the scientific and medical literature on topics of interest, but I decided to Google a few health-related topics that day. My googling brought me to Nat’s interesting day-by-day account of five days of drinking only water… No food and no drink (other than water) for five days.

Nat discussed “refeeding syndrome” as a risk and “where your reintroduction of food spikes your insulin so much that you go into shock and maybe die. It’s a concern on fasts five days or longer, and when you’ve lost a lot of body weight.”

Nat also noted, “a lot of the sites that talk about fasting say that you should start with fruits and vegetables since they’re easily digestible. Vegetables make sense, but fruit? Fruit has a fairly high glycemic index, meaning it risks spiking your insulin.”

This left me thinking, that’s not quite right. Insulin can play a role in refeeding syndrome, but it’s not the direct reason why refeeding syndrome is so dangerous. I reached out to Nat to explain how I understand refeeding syndrome, and surprise! – He answered back! It’s nice to converse with intelligent strangers on the web. In keeping with “nice stranger” etiquette, Nat encouraged me to share my explanation with others.

Here it is.

What is Refeeding Syndome and Should I Worry About It?

For a 5:2 diet, time-restricted eating, or alternate day fasting, the issue of how to “break” the fast, or begin eating again after fasting is a non-issue. The time without food isn’t long enough to raise concerns about refeeding syndrome. Refeeding syndrome is a potentially fatal shift in fluids and electrolytes and may occur in severely malnourished people when they begin eating again. Severely malnourished typically does not apply to someone coming off of an intentional, five-day fast.

The dangers associated with refeeding are of concern for severely malnourished, often hospitalized patients, when they begin receiving nutrition again. They may receive nutrition as enteral or parenteral nutrition. Enteral nutrition is given through a feeding tube. Parenteral nutrition, a highly specialized solution of nutrients, is given through a central vein in the body. For people who are able to eat food or drink liquids, refeeding would involve eating meals and snacks.

So, back to the comment about fruit and insulin spikes…

The negative effects of refeeding when a person has serious health issues or is severely malnourished aren’t directly related to insulin. Dangerous refeeding syndrome is related to the movement of fluids and electrolytes, such as potassium and phosphorus, within the body when they start eating or taking in calories again. Specifically, refeeding syndrome is caused by the movement of electrolytes from the intercellular (between cells) to the intracellular (within cells) space.

Am I At Risk of Refeeding Syndrome?

For otherwise healthy, well-nourished people, actual refeeding syndrome after a five day fast is very unlikely. If basic nutrition is good, and a person normally eats a well-rounded diet, five days of fasting is unlikely to cause the level of severe malnutrition associated with a risk of refeeding syndrome.

Very notable examples of widespread refeeding syndrome occurred during and after WWII. And the level of malnutrition associated with these events was severe. Consider survivors of the holocaust, who were literally starving to death in concentration camps. Or consider portions of the Dutch population who became malnourished when the German military blockaded portions of the Netherlands during WWII, resulting in widespread famine. These two groups are among those in which deaths due to refeeding syndrome were first observed in large numbers. It is this type of severe malnutrition associated with high risk of refeeding syndrome, but why?

When a person fasts, cells in the body begin to use stored energy for fuel. Glycogen, which is the storage form of glucose, is used first. Glycogen is broken down into glucose and released into the blood. Once this fuel is used up, usually after a day or two, the body begins to use fat and protein for fuel. If a person is otherwise healthy, most of the fuel (energy) your body needs will come from fat. The body will use some protein to keep basic metabolic functions going, but you will aggressively recycle amino acids – the building blocks of protein – to minimize loss of lean body mass. After five days of fasting, only small portions of your body stores of fat and protein will be used. This does not result in severe malnutrition.

However, as your body uses up and releases fuel into the blood stream, it also releases potassium, phosphorus, and other electrolytes that were in those cells. Again, these changes and shifts in electrolytes from within cells into the circulatory system are quite small after five days of fasting for an otherwise healthy person. The shifts only become life threatening after prolonged insufficient intake of energy and nutrients. Five days is not prolonged (though it feels like it if your intentionally fasting!).

Fortunately, the body preferentially uses up and destroys abnormal cells and components of cells, which is a good thing! This is called autophagy, and it is a healthy adaptation to fasting (for an interesting article on fasting and autophagy, try Can a Diet That Mimcs Fasting Turn Back the Clock?). Autophagy may be one of the key ways through which limited fasting improves health. But again, the autophagy occurring after five days of fasting is very small compared to autophagy occurring in people who are starving over weeks and months.

So what does autophagy have to do with refeeding syndrome? If you are severely malnourished, your intracellular – within cells – stores of potassium, phosphorus, and other electrolytes are very depleted. This is because your cells themselves are depleted. Your cells have, quite literally, “shrunk” as their contents have been released into the bloodstream for energy.

These cellular stores of electrolytes also are depleted because the small amounts of electrolytes you have left are in circulation. Your body must maintain minimum levels of these electrolytes in circulation to keep your heart beating and your brain functioning. Low levels of electrolytes in the blood can be life threatening, which is why these levels are checked daily when you are in the hospital. Your body will work very hard to keep these levels stable, even when you are starving.

This is where refeeding syndrome comes into the picture. Your electrolytes are very depleted. What little electrolytes you have are in circulation. When you “refeed” (via eating or enteral or parenteral nutrition), your body very rapidly starts shuttling the incoming energy (protein, fat, carbohydrates) into your cells to rebuild them. In order to rebuild your cells and tissues, the body also pulls electrolytes out of circulation back into cells. This movement of electrolytes from “between the cells” (in circulation) back into the cells can cause a dramatic and rapid drop in blood electrolyte levels.

It is the drop in blood electrolytes – potassium, phosphorus, calcium, and more – that leads to refeeding syndrome. This rapid electrolyte drop can stop your heart. In short, the heart muscle doesn’t have the electrolytes it needs to maintain it’s electrical pulses. It is the electrical pulses which cause the muscles to contract and your heart to beat. No electrical pulses = no heart beat. And no heart beat = death. If refeeding syndrome is severe, a person will, in essence, have a “heart attack.”

As for insulin, it does play a role in refeeding in an indirect way. Insulin is released when you take in calories, and is a signal to your body to begin shuttling those calories into cells to rebuild and repair them. In this way, insulin is the signal that starts refeeding syndrome, but insulin itself isn’t the main cause of this problem. Insulin triggers the cellular rebuilding, but it is the rapid drop in blood levels of electrolytes that is the main driver of refeeding syndrome.

How Would You Break-a-Fast?

Now you know what refeeding syndrome is (the basics – I’ve simplified it quite a bit!). You also understand, for otherwise healthy people who occasionally restrict food intake as a part of intentional fasting, refeeding syndrome is very unlikely. I say “very unlikely” because I am not willing to say it could never happen.

Anything can happen, which is why it makes sense to eat a light, balanced meal as your first meal after fasting. Try a piece of toast with peanut butter. Or a piece of fruit and a handful of nuts. Some greek yogurt with fruit is a good choice. Try a piece of lean fish or chicken and some vegetables. Whatever you eat, start slowly, allow your body to respond to the food, and go from there.

Most importantly, if you have a health condition or take prescription or over-the-counter medication for any reason, do not try fasting without first talking to your doctor. Going without calories for longer than normal may change how your body uses medication, or may not be safe for you for other reasons.

I admit, I used to think of fasting as a “fad,” with no potential health benefit. However, the emerging research has changed my mind, and you might want to learn about this fascinating topic too! Running on Empty, a nice overview article in The Scientist, highlights the many ways in which purposefully skipping food for hours to days may improve health.

According to Running on Empty, “periodic fasting reprograms T-cell populations, tamping down autoimmunity.” The author notes, “…lack of incoming calories appears to prune away autoimmune T cells…” and “…several fasting studies have also pointed to a decrease in inflammatory cytokines.” This should improve health for people with autoimmune conditions.

Can fasting help people with cancer? Preventing, managing, and curing cancer is another topic I’m passionate about. According to the article, intermittent fasting may make “tumor cells more susceptible to chemotherapeutic agents while protecting healthy cells from the treatment’s toxicity,…” Studies in mice support this effect, and human studies are being undertaken to see if this works for people with cancer too.

The article in The Scientist describes different types of fasting and discusses how intermittent fasting may be of benefit for preventing and managing other chronic diseases as well, such as heart disease, neurodegenerative diseases, diabetes, and more. Check it out and see if you find the idea of fasting for health as interesting as I do.

Please keep in mind, if you have a health condition or take prescription or over-the-counter medication for any reason, do not try fasting without first talking to your doctor. Going without calories for longer than normal may change how your body uses medication, or may not be safe for you for other reasons.